1. Field of the Invention
The present invention relates to materials and processes to prepare polycrystal and monocrystal forms for use in fuel cells in sensors and as catalysts. In specific embodiments, a fuel cell having oxygen/solid lanthanum fluoride (as a single crystal)/hydrogen configuration produces about 1 volt of open circuit potential at essentially ambient temperature. In other specific embodiments, specific mixed lanthanide or alkaline earth fluorides also produce electricity at moderate temperatures. Embodiments also include a porous perovskite-type metallic transition metal oxide electrode and a lanthanum metal/alkaline earth/fluoride electrolyte which are useful as a solid electrode/electrolyte in a fuel cell, as a sensor, or as a catalyst.
2. DESCRIPTION OF THE RELEVANT ART
Fuel cells convert chemical energy to electrical energy directly, without having a Carnot-cycle efficiency limitation, through electrochemical oxidation-reduction reactions of fuels. Several types of fuel cells have been or are being investigated at the present time. These may generally be classified as shown in FIG. 1, as Table 1, depending upon the kinds of electrolyte used and the operation temperature.
The solid electrolyte fuel cell which can be considered as the third generation fuel cell technology, is essentially an oxygen-hydrogen (or H.sub.2 --CO mixture) fuel cell operated at high temperature (ca. 1000.degree. C.) with a solid ceramic oxide material used as the electrolyte. At present, yttrium- or calcium-stabilized zirconium oxides have been used as the electrolyte. The mechanism of ionic conduction is oxygen ion transport via O.sup.2- anion in the solid oxide crystal lattice.
Additional references of interest include the following.
B.C. LaRoy et al., in the Journal of the Electrochemical Society: Electrochemical Science and Technology, Vol. 120, No. 12 pp. 1668-1673, published in December 1973, disclose some electrical properties of solid-state electrochemical oxygen sensors using vapor deposited thin films. Polycrystalline lanthanum fluoride solid electrolytes were investigated at ambient temperature.
T. Horiba in U.S. Pat. No. 4,550,067 discloses secondary cell batteries in which the positive electrode is made of materials such as phthalocyanine complexes, metal porphyrin complexes and the like.
Lyall in U.S. Pat. No. 3,625,769 and Fouletier in U.S. Pat. No. 4,526,674 each disclose lithium/oxygen fuel cells.
Raleigh in U.S. Pat. No. 4,118,194 and Weininger in U.S. Pat. No. 3,565,692, each disclose halogen electrochemical cells or the like.
Ledorenko in U.S. Pat. No. 4,172,022 and Eliot in U.S. Pat. No. 3,645,795, each disclose the use of phthalocyanine compounds in gas sensor electrodes.
Zeitner in U.S. Pat. No. 3,909,297 discloses a lithium-chloride battery.
In U.S. Pat. Nos. 3,698,955 and 3,719,564, Lilly discloses the use of rare earth fluorides such as lanthanum fluoride as solid electrolytes which are deposited as their films in a battery and a gas sensor respectively.
G. W. Mellors in European Patent Application No. 055,135 discloses a composition which can be used as a solid state electrolyte comprising at least 70 mole percent of cerium trifluoride and/or lanthanum trifluoride an alkaline earth metal compound e.g. fluoride, and an alalki metal compound e.g. lithium fluoride.
Additional reference cited also include the following.
B. V. Tilak, R. S. Yeo, and S. Srinivansan (1981), "Electrochemical Energy Conversion-Principles", in "Comprehensive Treatise of Electrochemistry" Vol. 3: Electrochemical Energy Conversion and Storage (J.O'M. Bockris et al. editors), pp. 39-122, Plenum Press, New York.
K. K. Ushiba, (1984), "Fuel Cells", Chemtech, May, pp. 300-307.
A. Sher, R. Solomon, K. Lee, and M. W. Muller (1967), "Fluorine Motion in LaF.sub.3 ", in "Lattice Defects and Their Interactions", R. R. Hasiguti, Editor, pp. 363-405, Gordon and Breach Science Publishers, New York.
A. Yamaguchi and T. Matsuo (1981), "Fabrication of Room Temperature Oxygen Sensor Using Solid Electrolyte LaF.sub.3 (Japanese)", Keisoku-Jidoseigyo-Gakkai Ronbunshu, Vol 17(3), pp. 434-439.
M. A. Arnold and M. E. Meyerhoff (1984), "Ion-Selective Electrodes," Anal. Chem., Vol. 56, 20R-48R.
S. Kuwata, N. Miura, N. Yamazoe, and T. Seiyama (1984), "Potentiometric Oxygen Sensor with Fluoride Ion Conductors Operating at Lower-Temperatures (Japanese)", J. Chem. Soc. Japan., 1984(8), pp. 1232-1236, and "Response of A Solid-State Potentiometric Sensor Using LaF.sub.3 to A small Amount of H.sub.2 or CO in Air at Lower Temperatures", Chemistry Letters, pp. 1295-1296, 1984.
M. Madou, S. Gaisford, and A. Sher (1986), "A Multifunctional Sensor for Humidity, Temperature, and Oxygen", Proc. of the 2nd International Meeting on Chemical Sensors, Bordeaux, France, pp. 376-379.
A. McDougall (1976), "Fuel Cells", Energy Alternatives Series (C. A. McAuliffe, series editor), The Macmillan Press Ltd., London.
T. Takahashi (1984), "Fuel Cells (Japanese)", Chemistry One Point Series 8 (M. Taniguchi, editor), Kyoritsu-shuppan, Tokyo, Japan.
N. Yamazoe, N., J. Hisamoto, N. Miura, S. Kuwata (1968), "Solid State Oxygen Sensor Operative at Room Temperature", in Proc. of the 2nd Int. Meeting on Chemical Sensors, Bordeaux, France.
J. Meuldijk, J. and H. W. den Hartog (1983), "Charge Transport in Sr.sub.1-x La.sub.x F.sub.2+x solid solutions. An Ionic Thermocurrent Study", Physical Review B, 28(2), pp. 1036-1047.
H. W. den Hartog, K. F. Pen, and J. Meuldijk (1983), "Defect Structure and Charge Transport in Solid Solutions Ba.sub.1-x La.sub.x F.sub.2+x ", Physical Review B, 28(10), pp. 6031-6040.
J. Schoonman, J., G. Oversluizen, and K. E. D. Wapenaar (1980), "Solid Electrolyte Properties of LaF.sub.3 ", Solid State Ionics, Vol. 1, pp. 211-221. A. F. Aalders, A. Polman, A. F. M. Arts and H. W. de Wijn (1983), "Fluorine Mobility in La.sub.1-x Ba.sub.x F.sub.3-x (O&lt;x&lt;0.1) Studied by Nuclear Magnetic Resonance", Solid State Ionics, Vol. 9 and 10, pp. 539-542.
A. K. Ivanovshits, N. I. Sorokin, P. P. Fedorov, and B. P. Sobolev (1983), "Conductivity of Sr.sub.1-x Ba.sub.x F.sub.3-x Solid Solutions with Compositions in the Range 0.03.ltoreq..times..ltoreq.0.40, "Sov. Phys. Solid State, 25(6), pp. 1007-1010.
J. O'M. Bockris, and T. Otawaga (1984), "The Electrocatalysis of Oxygen Evolution on Perovskites", J. Electrochemical. Soc., 131(2), pp. 290-302.
Additional general information is found in "Fuel Cells" by E. J. Cairns et al. in Kirk-Othmer: Encyclopedia of Chemical Technology, (3rd Ed.), Vol. 3, pp. 545-568; and in "Fuel Cells" by O. J. Adlhart in Van Nostrand's Scientific Encyclopedia, 6th ed., D. M. Considine (ed), Van Nostrand Reinhold Co., New York, pp. 1296-1299, 1986, which are both incorporated herein by reference.
Solid electrolyte fuel cells have several advantages over the other types of fuel cells:
1. There are no liquids involved and, hence, the problems associtated with pore flooding, maintenance of a stable three-phase interface, and corrosion are totally avoided.
2. Being a pure solid-state device, it poses virtually no maintenance problems. For example, the electrolyte composition is invariant and independent of the composition of the fuel and oxidant streams.
3. Inexpensive metallic oxides (ceramics) rather than expensive platinum can be used as the electrode catalysts.
4. The solid electrolyte fuel cells demand less feed gas preparation than the phosphoric acid cell (see FIG. 1), which requires a conversion of CO to H.sub.2 via the water-gas shift reaction, or the molten carbonate cell (see FIG. 1), which requires a carbon dioxide loop due to the use of carbonate ions for ionic transport.
The attraction of developing a solid electrolyte fuel cell is its simplicity. However, a high operation temperature (ca. 1000.degree. C.) is by far the most critical aspect of this type of fuel cell. Although high operation temperature produces high-quality exhaust heat that can generate additional electrical power, leading to a high overall system efficiency, maintaining the integrity of the cell components such as the interconnector is the most difficult challenge.
It is therefore desirable to develop alternative low temperature solid materials and composites for use as solid electrolytes in fuel cells, as solid sensors and as solid catalysts that can be operated in a range of 400.degree.-600.degree. C. or lower (preferably about 200.degree. C., especially at ambient temperature). Some of the structures described herein have been examined for usefulness as battery electrolytes. However, non of the references cited hereinabove, individually or collectively, disclose or suggest the present invention as described herein. The present invention relates to the design of such low temperature solid electrolyte fuel cells, sensors, or catalysts based on non-oxide solid electrolytes, such as solid solutions of lanthanide fluorides (e.g. La.sub.x Sr.sub.1-x F.sub.2+x).